The invention relates to a method for the locally resolved measurement of a radiation distribution produced using a lithography mask, e.g. within the framework of a projection exposure tool or a mask inspection device, an apparatus for the locally resolved measurement of a radiation distribution, a projection exposure tool for microlithography, and a mask inspection device for inspecting a lithography mask.
In order to measure optical systems in microlithography in respect of their imaging quality, one often resorts to the so-called aerial image measuring technique. The aerial image measuring technique is in contrast to a structure-producing measuring technique, wherein a measuring structure is imaged onto a photoresist layer of a wafer and the photoresist structure thus produced is then measured. With the aerial image measuring technique an aerial image sensor is used with which the intensity distribution of an imaged measuring object structure is detected, locally resolved, in the three-dimensional space in at least two directions, for example two lateral directions in relation to the optical axis of the imaging optics used. The measurement of the intensity distribution must not necessarily take place in air here, but it can also be implemented in a different gaseous or liquid medium or in a vacuum.
With the conventional aerial image measuring technique a basic distinction is made between imaging techniques and scanning techniques. With scanning techniques an aerial image sensor is moved mechanically within the three-dimensional space and detects the radiation intensities point by point at the corresponding points of the space. The aerial image sensor only measures one signal value here at the respective time. The measurement of an at least two-dimensional radiation distribution is therefore very time-consuming with this method.
In order to reduce the measuring time, radiation detectors with which electromagnetic radiation can be detected, locally resolved, are therefore often used. US 2006/0269117A1 thus discloses an aerial image detector for radiation in the extreme ultraviolet wavelength range (EUV) which generates an optical object in the visible light range from the EUV aerial image by using a scintillator. This is imaged, enlarged, onto a camera by high-aperture imaging optics. The resolution limit for these types of imaging objective is a function of the wavelength of the light used for the imaging in the far field, and with visible light is over 100 nm. This resolution is not high enough, however, for the qualification of modern EUV projection objectives or EUV lithography masks.
Furthermore, it is possible to introduce into the aerial image a solid body which emits photoelectrons and to detect the latter, spatially resolved, with an electron microscope, as described in WO 03/058344A2. However, this method requires a very high level of complexity.